191 



considers the idea of a great debacle, or diluvial current, winding 

 its way back in lazy flexures towards the point from which it 

 started, as absolutely unintelligible. 



If I might give my own opinion on this debated question, I 

 should say, that the existing river drainage of every physical region, 

 is a complex result, depending upon many conditions — the time 

 when the region first became dry land — its external form at the 

 time of its first elevation above the sea — and all the successive 

 disturbing forces which have since acted upon its surface. But 

 none of these elements are constant: no wonder, then, that results 

 derived from distant parts of the earth should be so greatly in con- 

 flict with each other. In the formation of valleys there is therefore 

 little wisdom in attributing every thing to the action of one modi- 

 fying cause. We know by direct geological evidence, that nearly 

 all the solid portions of the earth were once under the sea, and 

 were lifted to their present elevation, not at one time, but during 

 many distinct periods. We know that elevating forces have not 

 only acted in different places at different times, but with such va- 

 riations of intensity, that the same formation is in one country ho- 

 rizontal, in another vertical ; in one country occupies the plains, in 

 another is found only at the tops of the highest mountains. Now 

 every great irregular elevation of the land (independently of all 

 other results) must have produced, not merely a rush of the re- 

 tiring waters of the sea, but a destruction of equilibrium among the 

 waters of inland drainage. Effects like these must have been fol- 

 lowed by changes in the channels of rivers, by the bursting of 

 lakes, by great debacles, and in short by all the great phenomena 

 of denudation. In comparing distant parts of the earth, we may 

 therefore affirm that the periods of denudation do not belong to 

 one, but to many successive epochs ; and by parity of reasoning 

 we may conclude that the great masses of incoherent matter which 

 lie scattered over so many parts of the surface of the earth, belong 

 also to successive epochs, and partake of the same complexity of 

 formation. 



The excavation of valleys seems therefore to be a complex re- 

 sult, depending upon all the forces, which, acting on the surface of 

 the earth since it rose above the waters, have fashioned it into its 

 present form. We have old oceanic valleys which were formed at 

 the bottom of the sea in times anterior to the elevation of our con- 

 tinents. Such is the great valley of the Caledonian canal, which 

 existed nearly in its present form at a period anterior to the con- 

 glomerates of the old red sandstone. We have longitudinal valleys 

 formed along the line of junction of two contiguous formations, 

 simply by the elevation of their beds. To this class belong some 

 of the great longitudinal valleys of the Alps. We have other val- 

 leys of more complex origin ; where the beds through which the 

 waters now pass have been bent and fractured with an inverted 

 dip at the period of their elevation. Such is the valley of Kings- 

 clere, described in a former volume by Dr. Buckland. We have 

 valleys of disruption, marking the direction of cracks and fissures 



b 2 



